Alright so in d1 we learned Rasputin can control expos in a similar fashion as a submind, it’s an extension of him but still autonomous and independent, felwinter is his “son” in this way. While an exo in his first life felwinter died and we see this in the section of the lore and reads as “what’s going on,was I damaged in crypt processing? Who are you?” This is when he is also first resurrected by the light which left him with no memory which is what leads to the disconnection to Rasputin

[quote]I think that the lore revealed in the last step of “The Lie” quest was quite interesting, but I’m still left scratching my head over something. What exactly was Rasputin’s investment in Felwinter? What was it Felwinter did that made Rasputin single him out and hunt him down and destroy him? What is their connection?[/quote] Felwinter (pre collapse) was a Sidhartha Golem. These are Exo’s programmed to research information (mainly literature, art, music, media, etc) and give it to Rasputin in order for him to go on to possess an exo with all this info and become more human. So he could walk amongst humanity. Rasputin regarded him as a “son” and didn’t want what he had, all that info to fall out of his control. So after many failed attempts at killing him with warsats and frames. He led not just Felwinter, but all the Iron Lords to SIVA and killed them (except for Saladin Forge and Lady Efrideet who’s already left the order years prior).

Here’s the summery, warning for those who have not done the quest...l Felwinter was the “son” of rasputin, he was going to be used to talk to the surviving humans of the collapse, but felwinter died, and got resurrected, and became out of rasputins reach, rasputin got angy, and tried to kill him, cause if he can’t control felw, no one can... Eventually, rasputin tricked the ironwolves, to take on SIVA and sprang a trap to kill FELW

Thought the quest summed it up nicely.